The present invention relates to a zoom lens system having a zoom ratio of about 3 and including wide-angle and telephoto capabilities for use in a photographic camera.
Zoom lens systems of the type described above with a zoom ratio of about 3 are not suitable for use in the so-called two-unit type, or the retrofocus type having a negative first lens unit and a positive second lens unit in that the diameter of the second lens group is sharply increased. Zoom lens systems of this type ordinarily include four-unit type systems having a focusing lens unit, a variator lens unit, a compensator lens unit and a relay lens unit, and three-unit type systems composed of a focusing lens unit, a variator lens unit and a master lens unit which are all movable for performing both power variation and compensation.
Examples of the four-unit type systems are known from Japanese Patent Laid-Open Publication Nos. 63634/76 and 48607/81. Due to the limitation on the overall length (from the first plane to the image plane), however, the disclosed lens systems have a front lens of quite a large diameter to assure a required degree of brightness of the edge of the image field for an object at a short distance when the lens system is used as a wide-angle lens. To cope with this difficulty, the lens system of the Publication No. 48607/81 has an aperture provided in the compensator lens unit, but not in the relay lens unit, to allow the position of an entrance pupil to be closer to the object in the wide-angle lens mode, thereby reducing the diameter of the front lens. Nevertheless, the lens system cannot be rendered sufficiently compact as the overall length is fixed. Furthermore, since the power variation system up to the compensator is not an afocal system, the aperture needs to be opened or closed and moved along an optical axis to keep F number constant, resulting in a complex construction.
Japanese Patent Laid-Open Publication Nos. 30855/79 and 156912/80 show examples of the three-unit lens systems.